1. Field of the Invention
The present invention relates to laser peening, and more specifically, it relates to methods of laser peening a metal to prevent the absorption and adsorption by that material of deleterious atoms and molecules.
2. Description of Related Art
Hydrogen embrittlement is a major cause of metal failure especially fastener failure. The prevailing thought is that steels with a Rockwell hardness above C30 are vulnerable. Titanium and aluminum alloys are also susceptible to hydrogen reactions. The phenomenon is well known, although the precise mechanism has been elusive despite extensive research. A number of mechanisms have been proposed, and most are considered to have at least some merit. A widely held theory is that susceptibility to hydrogen embrittlement is related directly to the trapped population in the material. Generally, hydrogen embrittlement can be described as absorption and adsorption of hydrogen which promotes enhanced de-cohesion of the material (e.g., steel), primarily as an intergranular phenomenon.
Electroplating is a major cause of hydrogen embrittlement. Some hydrogen is generated during the cleaning and pickling cycles, but by far the most significant source is cathodic inefficiency, which is followed by sealing the hydrogen in the parts. Baking is often performed on high strength parts to reduce this risk. For the production plater, having to remove the parts from the production line to bake, followed by a separate chromating process, is a laborious process.
Hydrogen will penetrate into materials along grain boundaries and dislocations, hastening hydrogen embrittlement. Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. In nuclear power plants without aluminum components, hydrogen embrittlement is a primary reason that reactor coolant is maintained at a neutral or basic pH.
If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is transgranular. At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.
Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can re-diffuse from the steel, so that ductility is restored.
To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ (in position) hydrogen introduced during the service life of a part.
Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways. Some manufacturing operations such as welding, electroplating, phosphating and pickling facilitate hydrogen entry. If a material subject to such operations is susceptible to hydrogen embrittlement, then a final baking heat treatment is employed to expel any hydrogen. Hydrogen entry may be caused as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”. Hydrogen entry can result from the use of cathodic protection for corrosion protection if the process is not properly controlled.
Hydrogen embrittlement has been a problem for metals in a number of industries, especially in petroleum and gas production, where high concentrations of H2S are frequently encountered. Other hydrogen rich environments in which processes are employed, such as electroplating, picking, casting, corrosion and fuel cell reactions, have also been found to produce hydrogen damage in metals. Hydrogen atoms diffuse through the surface of metals into interstitial lattice sites or through microstructural channels such as grain boundaries, dislocation pile-ups, carbides, and defects. Once excessive hydrogen atoms are trapped and accumulated inside the metal several damage mechanisms may occur, including cracking, blistering, hydride formation, decarburization at elevated temperature and altering of tensile strength and lowering of ductility. Consequently, these deleterious effects lead to degradation of mechanical properties, and may result in eventual structural failure.
Austenitic stainless steels are employed in large quantities in these industries because of their high strength and good corrosion resistance. Their low diffusivity and high solubility of hydrogen in the FCC lattice structures provide generally good resistance to hydrogen damage in many aggressive aqueous environments. However, it has been found that with enough hydrogen permeation, austenitic stainless steels can become embrittled, and that brittle fracture has occurred in severe environmental conditions such as cathodic charging.
Hydrogen rich environments such as fuel cell reactors can exhibit damage caused by hydrogen permeation in the in the form of Hydrogen Induced Cracking (HIC) and Stress Oriented Hydrogen Induced Cracking (SOHIC). Both processes are known to decrease material ductility and can lead to corrosion cracking or failure in equipments. Although coatings and liners have been investigated by industry, there has been no definitive laser peening method referenced in literature with respect to preventing or retarding hydrogen embrittlement.